1. Field of the Invention
This invention relates to continuous flow membrane separation apparatus for use in osmosis, dialysis and electrodialysis and to a method of operating such apparatus. In particular, this invention relates to (1) a special assembly usable in such apparatus and comprising a tubular membrane and its associated reinforcing means and (2) an arrangement in which at least one flow channel chamber of the apparatus is separated from each of two adjoining flow channel chambers by, respectively, one of two separate membrane barriers having different molecular weight cutoff values and (3) an electrodialysis apparatus having at least four separate flow channel chambers.
2. Prior Art
Membrane separation is a process by which one or more components carried by a crude or feed fluid which is in contact with one side of a semipermeable membrane are separated from other components in the crude when a wash or rinse fluid is in contact with the other side of that membrane. Examples of membrane separation are direct osmosis, usually called simply osmosis, dialysis and electrodialysis.
In continuous flow membrane separation there has existed the problem of reinforcing the membrane, whether it be of the tubular type or the flat sheet type used in plate and frame apparatus. To maximize the permeate flux through the membrane, such membrane is usually made as thin as possible. To reinforce such a thin membrane against rupture under the pressure involved in producing the continuous flow, a reinforcing means in contact with the membrane is needed which perforce covers a relatively large portion of area of the membrane if the transmembrane fluid pressure difference is relatively large. The presence of such a reinforcing means thus significantly reduces the effective bare area of the membrane in contact with fluid on the low pressure side of the membrane, hence undesirably reducing the portion of the membrane from which permeate can flow unobstructedly away from the membrane and rinse fluid can flow unobstructedly against the membrane. Additionally, such a reinforcing means typically provides "dead" spots where fluid flow rate is reduced causing concentration polarization. Both of these phenomena thus reduce the effective flux through the membrane.
Typical prior art electrodialysis apparatus has three chambers, the chamber through which the crude or feed fluid flows being separated by membranes from each of the outer two chambers, each of which latter contains an electrode. In such an arrangement there is unavoidable contact between an electrode and the fluids, which may contain desired products, flowing in the chambers adjoining the single intermediate chamber containing crude. Such contact of electrodes with desired products is undesirable since it may well alter such products detrimentally. Also the products may detrimentally affect the electrodes by causing deposits thereon or the like.
By way of example, pertinent prior art is shown in U.S. Pat. No. 2,739,938 issued to S. G. Wiechers illustrating a three-chamber electrodialysis apparatus and in an article by Jan Al and S. G. Wiechers entitled "Production of an Artificial Human Milk", in the January 1952 issue of the journal "Research" subtitled "Science and its Application in Industry", published by Butterworths Scientific Publications, London, England. The article shows and describes the equipment of which the apparatus of the patent forms a part.
The apparatus of the patent comprises an inner, vertically oriented cylindrical electrode, an outer cylindrical electrode concentric with the inner electrode, and two tubular semipermeable membranes spaced from each other and interposed concentrically between the electrodes from which they are also spaced. This arrangement thus forms three concentric chambers, each annular in cross-section, through each of which liquid can flow in the vertical direction. The outer chamber is bounded by the cylindrical outer electrode and the outer cylindrical tubular membrane, the inner chamber is bounded by the cylindrical inner electrode and the cylindrical inner tubular membrane, and the intermediate chamber is bounded by the two tubular membranes. The patent teaches the use of flexible reinforcing means applied circumferentially around one or both membranes such as will give such an effective resistance to the stresses exerted on the membrane that the deformation of the membrane in the radial direction will remain negligible. In other words, the spacing between the strands of the reinforcing means is kept sufficiently close for such purpose. The patent notes that the liquid filling up the interstices between the strands or threads of the reinforcing means arranged close together will have a much lower velocity than in those regions through which the liquid can flow without impediment and that this slowing down is undesirable.
From the Al and Wiechers article it is clear that suspended solids, exemplified by flocculation of proteins (commonly present in liquids to be desalted by electrodialysis) tend to clog or foul the membranes.